A Theory of Stuttering

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What causes stuttering?

“We are born too soon to understand how the intermorphemic breaks in fluency we call stuttering are really produced, but it seems highly plausible that distortion in the total feedback signal system may be responsible for them.” (Van Riper, 1971, p. 395). To answer the question of how stuttering comes about is my aim on this website. I propose a theory of developmental stuttering, and the feedback signal system plays a crucial role in it.

Stuttering has been a subject of scientific research for a long time, and many theories of its nature and origin have been proposed. Today, thanks to new technical capabilities in brain research, we know much more about what is different in the brain of people who stutter, compared to normally fluent speakers. Additionally, some specific mutations were found in families in which stuttering has frequently occurred. But up to now, we don’t know how such neurological or genetic deviations make people, against their will, repeat words or parts of words, prolong sounds, or get totally stuck.

A plethora of empirical data about stuttering and about people who stutter was collected in the last decades without leading to a convincing, generally accepted theory. It is, however, impossible to logically derive theories from empirical data. Theories come about by thinking and modeling. Empirical data, on the one hand, can be suggestions that give thoughts a direction; on the other hand, they are bottom-up constraints confining the number of possible and plausible assumptions (read more).

The theory of stuttering presented on this website is a product of thinking and modeling, and any theory can be wrong. However, many features of the disorder are well known, and we can formulate questions about the causes of these features. A theory of stuttering has to answer these questions in a coherent and consistent manner. If a theory can do this, it should be considered a candidate for the correct theory.

Abstract

The role of auditory feedback (hearing one’s own voice) in stuttering has been controversially discussed for years. In many studies, alterations of auditory feedback were found to reduce stuttering; however, the mechanisms underlying those effects are hardly understood. On this website, I propose that persistent developmental stuttering is caused by insufficient processing of auditory feedback, which results in invalid error signals in the monitoring system; these error signals then interrupt the flow of speech. The processing deficit is ascribed to a misallocation of attention during speech. The theory accounts for the emergence of childhood stuttering, for the way stuttering becomes persistent, for the typical positions of stuttering events in sentences, and for the variability of the disorder depending on syntactic complexity, anticipation, and environmental factors.

The figure shows the core idea of the theory. The detection of a speech error (left) and stuttering (right) are based on the same mechanism; the internal monitoring system (in the circle) behaves equally in both cases. The disruption of auditory feedback (right) is thought to be caused by too little attention to the auditory channel during speech, that is, by too little perceptual or processing capacity for auditory feedback.

 
The main theses of the theory are:

• Auditory feedback provides information necessary for the automatic control of speech. This information must be processed by the brain on a sufficient level.
  
  
• The level at which the brain processes auditory feedback is modulated by the allocation of attention, reflecting the allocation of perceptual and processing capacities. If too little attention is directed to the auditory channel, feedback information is processed at a low level, insufficient for speech control.
  
  
• Stuttering is caused by invalid error signals due to poor processing of auditory feedback, resulting from misallocation of attention during speech. ‘Invalid error signal’ means that incomplete or noisy auditory feedback is misinterpreted as a speech error because the automatic monitoring system only compares expectation and perception and is unable to distinguish between a mismatch due to a real speech error and a mismatch because of a feedback disruption. The monitor responds to the error signal with an interruption of speech flow.
  
  
• The speaker is not aware of an error; therefore, he or she spontaneously tries to overcome the blockage of speech flow and to continue talking. Thereby, observable stuttering symptoms arise. The type of symptom – repetition, prolongation, or silent block – depends on the sound on which speech is interrupted, and on the speaker's behavior, such as speech drive and muscular tension.
  
  
• In most cases, the misallocation of attention initially arises because of the higher demands of speech planning when children start forming sentences. In some cases, particularly if stuttering onsets after preschool age, adverse, e.g., angst-burdened, communication situations or the memory of a trauma may have resulted in a misallocation of attention during speech.
  
  
• After a child has become aware of the disorder, anticipation and fear of stuttering and the attempt to avoid it can increase the misallocation of attention to the disfavor of auditory feedback, which can increase the severity of the disorder.
  
  
• Many aspects of the variability of stuttering are attributable to alterations in the allocation of attention. Whether one is speaking calmly or pressed for time, soliloquizing or talking to a large audience modulates the allocation of attention. Content words with a high information load and words with feared initial sounds are stuttered more frequently because they affect the allocation of attention.
  
  
• A deficit in the automatic control of attention due to a subtle anomaly in central auditory processing or a tendency towards hyperactivity or impulsivity (possibly generically caused) may cause a predisposition for developmental stuttering.
  
  

The above figure shows the causal chain underlying stuttering (AF = auditory feedback). The causal chain closes to a vicious circle if the experience of stuttering results in the anticipation or fear of stuttering, such that the speaker’s attention is more and more focused on the disorder during speech (see the figures below).

The figures show the vicious circle of persistent stuttering. Above: with some secondary behaviors that reinforce the vicious circle. Below: with factors (perhaps genetically caused) that may contribute to a predisposition for stuttering, and factors that can increase the frequency and severity of the symptoms (the circle has been rotated here only for reasons of depiction). The crucial component of the vicious circle is the allocation of attention; it is the interface between the mechanism underlying stuttering, on the one hand, and situational and emotional factors influencing stuttering severity, on the other hand.

The proposed theory implies (at least) four predictions:

• (1) The cerebellum plays a crucial role in stuttering. This is consistent with the results of Wymbs et al. (2013) and with reports of cases in which lifelong stuttering disappeared after impairment of cerebellar function (see Section 2.2);
• (2) Attention to auditory feedback determines whether auditory feedback is involved in motor control;
• (3) Normal speakers automatically pay attention to auditory feedback, but stutterers do not. The predictions 2 and 3 were confirmed experimentally by Lazzari et al. (2024); see also my comment on the study.
• (4) Amplified auditory feedback reduces stuttering. This was confirmed experimentally by Fiorin et al. (2021); see also my comment on the study.
  
  

This diagram corresponds to Fig. 1 in Fiorin et al. (2021). It shows that amplified auditory feedback reduced the percentage of stuttering-like disfluencies (SLD) more than DAF and white noise did. Unlike DAF and noise, there can be no doubt that amplified auditory feedback attracts the speaker's attention.

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